The overwhelming consensus online is that microwaving a babies bottle can be "dangerous" because microwaving formula can produce hot spots. However, if you simply swirl the bottle you can even out the heat distribution and eliminate said hot spots. The amount of bubbles produced by swirling is minimal and heat redistributes very easily in liquid. That being the case, if I test the formula's heat and its a good temperature, and the heat is redistributed, whats the big deal about microwaving the bottle? Is it just that people can't be trusted to swirl the bottle well and test the temperature?

Anticipating the argument; if your afraid of the plastic bottle giving off chemicals, then its also unsafe to sanitize them in the microwave.

I'd be as much or more concerned about breaking down the nutrients in the formula. Hot spots mean that you might have some portions at boiling at one point, which would denature proteins and might decrease the nutritive value of the formula.
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JoeMay 27 '14 at 14:27

So, I and one other person have given you an answer detailing why you shouldn't microwave bottles and heating solutions that avoid the downfalls, and you insist on refuting a minor (moot) point. Are you looking for an answer or an argument?
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NoahMay 27 '14 at 20:28

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For powdered formula, keep in mind that there is no damage done to the water when just heating the water prior to mixing, and mixing will certainly distribute the heat evenly. If you must use a microwave to heat your formula, then simply heat the water in the microwave prior to adding the formula. When you add the formula and mix it, you will have an evenly heated formula, and no damage from hot spots will have occurred to the nutrients.
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Adam DavisMay 28 '14 at 1:27

@Noah sorry if thats the impression you got. I appreciate your response, I would just like to have evidence if any exist that demonstrates that microwaving can significantly reduce the nutritional value of the formula. If it exist id like to believe it. +1 for your response.
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n00bMay 30 '14 at 15:54

@n00b Like I said, it's nothing special about microwaves, it's the nature of cooking, itself. Microwaves cook small portions of the formula, which is why heating methods are preferred to cooking methods.
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NoahMay 30 '14 at 16:37

6 Answers
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Depending on the formula that you're using, they might have some probiotics in their composition. For example, Nestlé.

Probiotics have small tolerance to high temperatures, so preparing the formula (or heating it after it was prepared) above some temperature might kill those probiotics, reducing the nutritional value of the formula. For example, this preparation guide advises to use water at 40 degrees Celsius (after it was boiled and let to cool down).

Using the microwave might require you to have a control over this temperature (i.e., you should be sure it won't get higher than 40 degrees). If specific points of the formula over-heat, in that area those probiotics might die, but it doesn't mean that the whole formula lost it value.

When it comes to your child's safety, is it really worth taking the risk? I believe a better statement would be "During the heating or feeding process, no single part of the bottle or food should ever be hotter than what you would give to your infant".

Swirling the formula around to distribute the heat is explicit acknowledgment that some part of the formula will be hotter than others, and likely hotter than you'd give to your infant. Additionally, the bottle itself might be unevenly heated, and in that case, the heat will not equalize nearly as quickly.

Piggybacking on what Joe mentioned, you're also risking breaking down the nutritive value of the formula by microwaving it.

To counter your point re: microwave sanitization: When you sanitize bottles in a microwave, they're all pointing downward and draining any fluid (and chemical) buildup. When you're microwaving a bottle with formula, there's no such drainage, and any chemical buildup is absorbed by the formula.

Potential Solutions

If you're using powder-mix formula, use warm water to mix instead of cold water.

If you're using full-strength formula, most don't need to be refrigerated, so they can stay at room temperature

If you're using a concentrate to mix pitchers and put the rest in the refrigerator, the best way to heat it is to run the body of the bottle under warm water, swirling the formula around so it all gets heated evenly.

Generally, if you have access to a microwave, you have access to hot water.

FWIW, we use powder, and pre-make (some) formula (boiled water) then store it. I would be interested to see any studies linking microwaving to destroying nutritional value, because it sounds possible, but unlikely I would think after 30 seconds of microwaving.
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n00bMay 27 '14 at 15:02

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It's not necessarily the microwave itself that causes the loss of nutrients, but the small pockets of formula that could reach temperatures above recommended serving temperatures, and potentially above boiling point. Any cooked food loses some nutrients, but even/thorough heating to serving temperature through warm water isn't "cooking" in the same sense, but microwaving is, for at least some parts of the formula.
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NoahMay 27 '14 at 15:25

As for this: "Swirling the formula around to distribute the heat is explicit acknowledgment that some part of the formula will be hotter than others"... yes. That is also why I stir my spaghetti sauce and black bean soup: some parts of it are getting hotter than others, even too hot in those spots to serve to adults. So the stirring evens out the heat. Sounds like a pretty tried-and-true technique when it comes to food... safe enough for my kids, at least.
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Dan HJun 1 '14 at 3:18

It's also why health professionals and manufacturers of baby food products advise against microwaving formula. See notes regarding the loss of nutritive value in formula due to cooking.
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NoahJun 1 '14 at 3:22

Noah & woliveirajr are both correct that there is some risk of reducing nutritional value, I'm sure.

I will offer as counter point my home study (sample size=2) of children who were bottle-fed formula almost exclusively as infants; formula which was on occasion reheated in the microwave. They are now 12 and 9, healthy, and not malnourished. They definitely suffer from one side effect often linked to formula feeding, which is being extraordinarily picky, bland eaters to this day. While the formula feeding may have contributed to that problem, I'm hard pressed to think how occasional microwaving of the formula would have made it worse - does killing nutrients also mean making it blander? Fail to nourish developing taste buds?

It's long enough ago that I don't have a firm recollection of just how often we did this. Certainly not all the time, but probably at least one feeding a day? But I could totally be making that up.

You're going to be an awesome part of this site, Thomas! This is, of course, anecdotal evidence. The question is, in all truth, too vague -- what the OP means by "ok" seems to be, "will it kill"? My great grandmother chewed tobacco until she was 97 and laughed about it. People have been using LSD for 76 years, many claiming no side effects. What is ideal? Just too hard to answer. What did I do and am still here to tell about? Much easier. What's "ok"?... impossible to answer as there is no universal definition.
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Jeremy MillerMay 28 '14 at 0:10

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+1 At about 6 months I'd enough of trying to carefully heat night bottles for our son while stupefied from lack of sleep, and started microwaving them. I reasoned that any loss of nutrition would be minor given that at the time the recommendation was to make bottles with boiling water. After microwaving I'd shake the bejesus out of the bottle to mix everything up and negate any hot spots. We've a happy healthy little boy with no nutritional problems and no eating problems. I think the "don't microwave" directive exists because "people" en-mass can't be trusted to think sensibly, or at all.
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Binary WorrierMay 28 '14 at 11:43

We microwave our infants formula all the time, it's all we use to heat the formula. We always swirl and double-check the temp before feeding. I disagree with the losing nutritional value statement. I don't think that warming milk to 20 degrees celcius is considered "cooking" to anyone. There is a fine line in microwaving time to go from feeding temperature to cooked milk, so you have to be careful and get used to it. But for us now we have the prep times all figured out. 4oz bottle 30 secs from the fridge, 19 secs from room temp for our microwave. Obviously times will vary from different devices. My wife and I had a "milk race" one night to see who could prepare the milk the fastest. She used the microwave, I used the bottle steamer. By the time my bottle was ready she had the baby half ways through the feeding with her bottle. Shame to say I lost. Typical husband I am...

You can also let it stand for a minute to help distribute heat. We did this with our kids, but never made the formula more than warm, and over time, made it less warm until we were actually serving it cold. There is nothing to say it has to be warm - it is transitioning from breast that makes it desirable for it to be warm.
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MJ6Dec 7 '14 at 21:10

I would occasionally microwave a bit of formula, especially to take the chill off if we had placed an unused portion in the fridge. (Yes, I will admit that publically... especially since I don't think my wife is on this forum... but I digress.)

One accommodation I made was this: I placed the bottle in a larger container, like a Pyrex measuring cup. I filled the cup with water a bit higher than the level of formula in the bottle. Then I microwaved the bottle in the surrounding water. I figured this would act to moderate the rate of increase in temperature, and help even out the "hot spots" (if any).

Upon removing from the microwave, I would test the water in the cup (with my fingers) for its temperature. If IT was not too hot, I figure there was a good chance the formula was not, either. I would then "swish" the bottle (as the OP mentions) and test it for temperature, too, by squirting on the wrist, or -- more reliably -- just tasting it.

The standing wave wavelength of microwave ovens is ~6.4cm. Add as much water as you want, it doesn't change the interference pattern
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NoahMay 30 '14 at 18:22

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Thanks for the science demo. Fortunately, my microwave does have a fully functional turntable, and thus my food (and baby bottle) moves in and out of all the "hot spots" -- which was one of the very points of the video you cite. Also, the water bath I propose does not serve to break up the standing waves but rather to act as a thermal mass to distribute the heat itself. By the way, consumer-grade microwave ovens usually use a frequency of 2.45 gigahertz -- which has a wavelength of 12.2 cm, not 6.4. But thanks for your concern.
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Dan HJun 1 '14 at 3:11